The large size of the vesicles of beige mouse peritoneal mast cells (4 microns in diameter) facilitated the direct observation of the individual osmotic behavior of vesicles. The vesicle diameter increased as much as 73% when intact cells were perfused with a 10 mM pH buffer solution; the swelling of the vesicle membranes exceeded that of the insoluble vesicle gel matrix, which resulted in the formation of a clear space between the optically dense gel matrix and the vesicle membrane. Hypertonic solutions shrank intact vesicles of lysed cells in a nonideal manner, suggesting a limit to the compressibility of the gel matrix. The nonideality at high osmotic strengths can be adequately explained as the consequence of an excluded volume and/or a three-dimensional gel-matrix spring. The observed osmotic activity of the vesicles implies that the great majority of the histamine known to be present is reversibly bound to the gel matrix. This binding allows vesicles to store a large quantity of transmitter without doing osmotic work. The large size of the vesicles also facilitated the measurement of the kinetics of release as a collection of individual fusion events. Capacitance measurements in beige mast cells revealed little difference in the kinetics of release in hypotonic, isotonic, and hypertonic solutions, thus eliminating certain classes of models based on the osmotic theory of exocytosis for mast cells.